This application claims the priority of German patent document 103 42 146.7, filed Sep. 12, 2003, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method of monitoring a fuel cell unit for detecting defects therein.
German patent document DE 43 38 178 A1 discloses an arrangement for monitoring the condition of fuel cell units, in which the fuel cells are connected sequentially in at least two parallel switched rows, each having the same number of cells, and in which case, the rows are divided into branches of a bridge circuit and are connected with at least one analyzing arrangement. The latter evaluates the voltage or the current tapped between the branches and generates a fault report in the event of deviations beyond permissible limits.
Methods of monitoring fuel cell stacks are also known from German patent document DE 195 23 260 A1 and International patent document WO 91/19328. There, an average value is determined for the measured voltages of the cells, and compared with the individual voltages of the fuel cells. When an individual voltage is lower than the average value by a predefined amount, a corresponding warning is emitted. In German patent document DE 195 23 260 A1, the difference between the highest and the lowest individual voltage is also determined and a warning is emitted when it exceeds a predefined limit value.
One object of the invention is to provide a method of monitoring a fuel cell unit, by which a faulty condition of the fuel cell unit can be detected early, so that preventative maintenance measures can then be taken.
This and other objects and advantages are achieved by the method according to the invention, in which a measured voltage value determined at the outputs of the fuel cell unit, (as part of a pair of measured values consisting of a measured current value and a measured voltage value) is compared with a limit value which is a function of the measured current value associated with the voltage value. The functional relationship between the limit value and the measured current value is given by a limit characteristic polarization curve of the fuel cell unit. (Here, a fuel cell unit, also called a fuel cell stack, may be constructed of one or more fuel cells.)
In the following, the term “current” will also include quantities related to the current, such as a current density. Measured values in the following will include values actually measured by means of suitable sensors as well as values which are defined by an estimation method (for example, by means of a Luenberger Observer).
The so-called characteristic polarization curve reflects the technical condition of a fuel cell or fuel cell unit. The characteristic polarization curve describes the current-voltage characteristic of a fuel cell or of a fuel cell unit.
As an example, FIG. 1 shows such a characteristic polarization curve or current-voltage characteristic. The measured voltage U of the fuel cell unit is entered as a function over the current density S of the fuel cell unit. The characteristic polarization curve illustrated in FIG. 1 shows three characteristic ranges, through which a fuel cell unit can pass. The terminal voltage, which can be tapped at the fuel cell unit, is typically reduced by overvoltages when the circuit is closed. In the range of low current densities, a “pass-through overvoltage” limits the voltage by the finite speed of the charge transfer at the so-called three-phase limit and the adsorption and reaction of the particles. In an “activation range”, this has the result that the voltage drops rapidly at low current densities. In the range of mean current densities, the resistance of the electrolyte and all other electron- and ion-conducting paths is responsible for a voltage drop which is largely linear with respect to the current density. In this range, normally called the “ohmic range”, the voltage drops less rapidly with respect to the current density than in the activation range or in the “saturation range”. In the saturation range at high current densities, the mass transfer effects—for example, concentration gradients because of an insufficiently fast diffusion of the reacting gases through pores or of the ions through the electrolyte—limit the characteristic polarization curve. The working range of a fuel cell or of a fuel cell unit is typically within the ohmic range.
The characteristic polarization curve represents a macroscopic description of the fuel cell unit. Microscopic effects, such as local current flows within individual cells, can be taken into account by an additional detailed modeling, for example, by means of the least error square and/or neuronal networks method. Detailed physical and/or chemical models can also be used which also model the local effects, such as the current distribution, the temperature distribution.
The characteristic polarization curve is used as a parameter function which describes the macroscopic condition of the fuel cell unit. Reversible and/or irreversible effects, such as contaminations, chemical and/or physical effects (for example, deposits, decompositions, erosions, dirt) have an indirect or direct influences on the course/the shape of the characteristic polarization curve. As a result, the characteristic polarization curve for one and the same unit is variable.
As an example, FIG. 2 shows a first characteristic polarization curve PN(I) and a second characteristic polarization curve PG(I) over the current I. The first characteristic polarization curve PN(I) (also called the “starting” characteristic polarization curve) describes the starting or new condition and/or the ideal condition of the fuel cell unit. The second characteristic polarization curve PG(I) (referred to as a limit characteristic polarization curve) describes the condition of the fuel cell unit starting from which the fuel cell unit is considered faulty and a repair becomes necessary. With respect to the voltage values, the limit characteristic polarization curve for the same current values is below the starting characteristic polarization curve. Deterioration is illustrated in FIG. 2 by an arrow.
The method according to the invention relates measured actual flow and voltage values to the limit characteristic polarization curve. On the basis of the actual values of current and voltage, a conclusion is drawn concerning the condition of the fuel cell unit. In a further embodiment of the invention, the relationship between the measured values and the starting characteristic polarization curve is also taken into account.
The method according to the invention has the advantage that the technical condition of the fuel cell unit can be continuously described and monitored. A diagnosis of the fuel cell unit is carried out, and even fast deteriorations and faults in the operating characteristics of the fuel cell unit can be detected early. A maintenance of the fuel cell unit can therefore be planned early and can already be carried out preventively.
The method according to the invention can be easily integrated in an analyzing unit, such as a control unit, and requires little storage space.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.